1. Field of the Invention
The present invention relates to an optical transmitter
2. Related Prior Art
Generally, an optical transceiver includes a light-emitting device and a light-receiving device within each assembly, and these assemblies are installed in a package accompanied with an electronic circuit. Moreover, in an optical communication system configured with the multi signal channel, a multi-channeled optical transceiver is applied. Japanese Patent published as JP-H05-091056A and JP-2002-111601A have disclosed typical multi-channel transceiver.
The multi-channeled optical transceiver generally provides subject such as (1) to reduce power consumption, and (2) to decrease a count of lead pins in assemblies installing the light-emitting device and the light-receiving device. In the optical transceiver disclosed in aforementioned Japanese Patents, JP-H05-091056A and JP-2002-111601A, the power consumption reaches at least N×V0×ILD, where N, V0, and ILD are the count of channels, the power supply voltage, and the average current flowing in the light-emitting device, respectively.
One method to reduce the power consumption has been disclosed in Japanese Patent published as JP-2003-188824A, in which the optical transceiver installs a transmitting optical subassembly (TOSA) including a semiconductor laser diode. This laser diode within the TOSA is driven by a paired transistor constituting a differential circuit with a common emitter. One of paired transistor, not connected to the laser diode, is used to supply a portion of the bias current of the laser diode. Thus, the power consumption can be reduced.
Assuming a case that the multi-channeled optical transmitter is configured based on a optical transmitter shown in Japanese Patent published as JP-2003-188824A, the total power consumption P0 of the multi-channeled optical transmitter becomes;P0=N*V0*IH,where N, Vo and IH are the count of channels, the power supply voltage, and the bias current for the LD when the LD turns on, respectively. In this case, the LD is driven by the differential circuit, namely, the LD is configured as a load device for a transistor constituting the difference circuit. The power consumption above becomes lower than that when the LD is driven by a conventional driver, in which the power consumption becomesP0=N*V0*(IH+IL),where IL denotes the bias current for the LD when the LD turns off.
Generally, the low-level current IL is merely 10˜20% of the high-level current IH. Moreover, the former current IL is determined by the threshold current of the LD, and recently developed LDs have relatively smaller threshold current. Thus, the reduction of the power consumption of the transmitter becomes harder in optimizing the structure and the material of the LD.
Therefore, the present invention is to provide an optical transmitter that installs a plurality of LDs and reduces the power consumption thereof.